1. Field of the Invention
Embodiments of the present invention relates generally to films, compositions, articles and methods for making same, where the film compositions comprise anti-reflection three dimensional (3D) porous films and/or coatings. In certain embodiments, the film compositions comprise nanostructured films and/or coating. The film or coating substrates can include plastics, polymers, glasses, ceramics, metal oxide solids such as silica, and other transparent, semi-transparent, non-transparent non-metals, metal oxide sheets and/or glasses.
More particularly, embodiments of the present invention relate to compositions and methods for preparing three dimensional (3D) anti reflection (ATR) porous films, coatings and/or layered compositions, which can be deposited on substrates or substrate surfaces, where the compositions have optical transmittance values of at least (greater than or equal to (≧)) 86% or reflection values of at most (less than or equal to (≦)) 4% and where the film compositions comprise 3D polymeric networks including polymers (e.g., polyelectroytes), oligomers and/or monomers in the absence or presence of polymerization initiators, crosslinking agents, and particles in desired ratios to achieve desired composition properties and characteristics. The compositions can then be cured via crosslinking and/or template particle can be dissolved or removed from the compositions to form more open 3D polymeric network films or coatings. This invention is particularly well suited for use in the fabrication of multilayered displays, packaging, optical parts, and other applications that involves viewing articles at various viewing angles and optimized light transmission. Other applications include forming composite compositions that combine anti-reflection film composition with electrostatic coatings, self-cleaning surfaces, catalytic surfaces, and sensing layers.
2. Description of the Related Art
Antireflection coatings are widely used to reduce the surface reflection of optical devices, display screens, photovoltaic devices, etc. Poisson and Fresnel defined the phenomenon as destructive interference between the light reflected from a substrate and the light reflected from a thin film coating that substrate. The air-film interface reflection for normal incident light can be described by the simplified Fresnel's equation:R=(n2−n1)2/(n2+n1)2 where n1, equal to 1, is the refractive index of air, and n2 is the refractive index of the film. For normal incident light, an ideal homogeneous single-layer antireflection coating should satisfy two conditions: 1) the thickness of the coating should be λ/4 where λ is the wavelength of the incident light, 2) nc=(na×ns)0.5, where nc, na and ns are the refractive indices of the coating, air, and substrate, respectively Natural and synthetic materials having such low refractive indices are either rare, expensive to obtain or to synthesize, or difficult to manipulate in thin film form.
However, porous or microscopically layered dielectric materials can easily achieve an effective refractive index approaching 1.23. For a porous film, the effective refractive index is given by an average over the film if the pore size is smaller than the wavelengths of interest Low refractive index metal oxides, such as SiO2, which could be negatively charged can be suitable materials for generating a porous film of lowered reflectance on a surface. In terms of characterization, absolute reflection can be obtained using an integrating sphere set-up, whereas simple absorbance/transmittance measurements can be made to characterize anti-reflection properties. In general, as the transmittance increases towards 100%, an increase in anti-reflection properties is observed.
There is an increasing interest in the layer-by-layer template assisted assembly. The fact that a large variety of different materials can be layered in controllable thickness and in desired order makes this approach almost universal especially with the growth of applications since it was first introduced by Decher and coworker in the early 1990s. The materials typically used in layer-by-layer assembly are small organic molecules or inorganic compounds, macromolecules, biomacromolecules or even colloids. The adsorption of alternating polyelectrolytes or nanoparticles is based on electrostatic or other non-covalent inter-molecular forces.
The layer-by-layer technique is thus a good candidate for a simple, and relatively fast, environmentally benign, and potentially economical process for generating anti reflection coatings. For multilayers containing multiple interfaces, multiple reflections take place. In such cases, a matrix theory can be applied. In this theory, the layers are modeled using a stratified medium theory and the light is assumed to be either s- or p-polarized. The overall effect of layers on reflectance of the surface takes the function of each layer into account To fabricate an antireflection coating, the layer-by-layer method can be utilized but is much slower than spin coating. It consumes a bulk volume of the chemicals even though the layer-by-layer method provides accurate control of the deposition density and thickness of the film.
Spin coating is yet another widely used process for applying a thin, uniform film to flat substrates. Final dry film thickness can be predicted using a model in terms of the primary process variables, spin speed, and initial polymer concentration After applying a simple approximation using a similarity boundary-layer analysis, the final film thickness can be approximated to be directly proportional to the number of unity, initial polymer concentration, one fourth of the multiplication of the kinematic viscosity and solute diffusivity, and to be reversibly proportional to the square root of spin speed.
A large number of applications of spin coating in anti reflection films have been reported, and the porous structure of film is a key factor to form an anti reflection film. Steiner and coworkers first reported the nanophase-separated polymer films as high-performance antireflection coatings using spin coating technique. More recently, spin-coated block copolymer film with sponge-like nanoporous structures have been reported followed by the first highly porous spin-coated polymer latex film. The porosity of film is affected by spin speed and by the concentration and particle size of polymethylmethacrylate latex. Different approaches of incorporate silica nanoparticles in the spin coatings have been reported Silica-incorporated layer-by-layer films show that a single layer of silica nanoparticle-incorporated film does not result in a large transmittance increase. Only after a specific number of silica nanoparticle layers, the porosity and film thickness of the layer-by-layer film are enough to generate significant improvement in transmittance.
Patent JP 06172428 shows that in the presence of aqueous NaOH, polyacrylic acid (PAA) is crosslinked by AlC3 solution. A metal ion bridge formed as the polymer is settled. PAA and polyvinylalcohol (PVOH) have been intensely studied for their intermolecular crosslinking both chemically by esterification with specific treatments and physically by forming H-bondings. At room temperature H-bonding between PAA and PVOH predominates.
Thus, there is a need in the art for a novel and facile method for making three dimensional anti-reflection (ATR) porous films onto substrates from porous 3D network films, nanocomposite structures, 3D structured gels, 3D structured hydrogels, or 3D structured aerogels comprising polymers, oligomers, monomers, polymerization initiators, cross-linking agents, particles, optionally template particles or materials and a solvent system.